I found a work-around. If you enter the address as:

POBOX 1234

City, State 56789

It will automatically offer you 2 suggestions to choose from for the correct address.

Choose the one that reads like this:

PO Box 1234

City, State [removed]

It will allow it since it is a correctly recognized postal address. 

I hope this helps! 

I entered the POBOX# no spaces and it worked. Thank you

To bad QB can't just fix this issue and then we wouldn't have to go looking for a fix to a simple problem.  There are PO Boxes everywhere, and are used by many people.  QB - FIX THIS ISSUE

Try Adding PO BOX (with capital for the word Box)  instead of PO Box and it should work